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The Shortarse Feelerfish and other conundrums

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16 June 2017 - 7:37pm

Presenting the Shortarse Feelerfish: the second specimen to be found in Australia. The scientific name, bestowed in New Zealand, is Bathymicrops brevyanalis (Bathymicrops for deep small eye; breveanalis for short anus). The name probably refers to the short anal fin rays that characterise this species. It belongs to the same family as tripodfish, and like them, has small vestigial eyes. Image: Alastair Graham, CSIRO Australian National Fish CollectionMartin Gomon greets a new tray of fish on the RV Investigator. Image: Asher Flatt

Day 33: Martin Gomon, Museums Victoria

What features stand out when you look someone, or something, new in the face . . . eyes, nose, mouth? Not in the abyss! For this is the place of the featureless face, where perhaps the most vital innovations of vertebrate anatomy are all but useless, with none being more so than the most powerful of them all, the eyes.

Why hold onto eyes where a reason for them doesn’t exist? Why have eyes where colour, form and texture can’t be conveyed because there is no light to convey them? Bioluminescence you say? It is indeed present, but not in great enough concentrations to allow vision of form and texture. And colour? No, not so much, because all the living fish are dark, pale . . . or translucent.

The Blind Cusk Eel is a small fish with transparent, gelatinous skin and no scales that looks more like a larvae than a adult fish. The skeleton is only partially calcified, and the muscles and gills are underdeveloped. The eyes and nasal organ are reduced, and they lack a swim bladder. They live at 2000 to 6000 metre depths. Image: CSIRO Australian National Fish Collection

A face without obvious eyes? Think the youngest, the just born, the pre-born whose eyes are still covered by skin, the nose little more than two pores at the front, the mouth hardly large enough to allow in water, much less food, a reasonable description of fishes in the abyss in general. Not just the one-off, but a story repeating itself again and again. In the video below, John Pogonoski talks about a classic example of a featureless fish: a gelatinous eel caught on the RV Investigator voyage.

The loss of functional eyes is a bit extreme you might say but the answer to that is in another question: “Why retain such expensive, energy eating organs, whose sophisticated cells would normally convey and interpret vital messages of sight about predators and prey, when the light’s not there?” No light, no sight, no messages. And why return to the form of the youngest?

Tripodfish have tiny eyes and very poor vision. Image: Robert Zugaro

Scientists might say these fishes have regressed to an ancestral stage, before evolution has imposed the cost of developing and maintaining an expense now no longer needed. Even the most elegant designs down here, like those acquired by the icons of this world, the tripod fishes on their tall spindly fins . . . but tiny, non-functional eyes . . . . have made their savings where they can.

And then, of course, there’s the odd one out of the group, the bottom-hugging Grideye Spiderfish with flat oval plates in the place of eyes, the reason for which still requires explanation. It certainly isn’t for sight in the way we think of it.

Grideye Spiderfish have a pair of large, greenish yellow oval plates on the upper surface of the head replacing eyes. The eye plates may act to detect bioluminescent prey or function as luminescent lures. Image: Asher FlattAnd of course there's the Blind Cusk Fish, now recognised around the world, despite having no face! Image: Asher Flatt

This is fascinating. Anxious to hear about future results. Question: Do eyes on these deep water critters perhaps function in a different capacity than what we define as how "eyes" function? FYI I have no background in any of this.

This is a subject that has not been well studied. Based on the form of the tiny eyes in many of the species, one assumes that they are little more than the remnants of the eyes of their sighted ancestors that lived at shallower depths. The eyes of some abyssal fishes could be the earliest form of eyes as found in extremely young individuals, which simply cease developing well before birth, or in those that have fully developed, but tiny eyes, cease to grow because there is no reason to have eyes that require a refined image. The larvae of fish in shallow illuminated environments are virtually fully developed at birth and grow proportionally as the fish grows. The very odd eyes of the Grideye Feelerfish lack a lens and are therefore incapable of forming an image as with"normal" eyes, but they do appear to be able to detect light. How they function is yet to be determined.